The present invention is concerned with improvements in borehole logging methods and apparatus, and, more specifically, a method of obtaining a log of a borehole with apparatus having at least one source of gamma radiation and at least one gamma radiation detector.
The usefulness of so called gamma-gamma well logging instruments for obtaining indications of earth formation density surrounding a borehole is well known, and the principles on which they operate are, similarly, well known to those skilled in the art. Generally, such density logging instruments take advantage of the Compton scattering effect, a phenomenon by which gamma radiation is scattered in an interaction with free or loosely bound electrons in the scattering medium. The gamma radiation scattering thus achieved can be measured in that when the energy of the incident gamma radiation is known, the energy of the gamma radiation that is Compton scattered at a particular angle with respect to the incident radiation can be calculated. Furthermore, the probability that a gamma ray will be scattered from a particular zone or volume of an earth formation is proportional to the number of electrons in the zone. The count rate of detected, singly scattered gamma radiation is, therefore, related to the density of the scattering zone.
Previously, attempts to measure the number of such singly scattered gamma rays that are scattered at a selected volume in an earth formation have produced collimation schemes to limit the emission and/or detection patterns for gamma radiation in the formation. Typical of these schemes are the techniques disclosed in the following U.S. Pat. Nos. 2,934,652; 3,202,822; 3,263,082; 3,321,627; 3,840,746; and 3,846,631. Such previous techniques, however, have failed to provide a collimation scheme with sufficient resolution capability to assure that the volume of interest was in fact the volume that produced the detected results. Furthermore, the prior art devices have, due to detection count rate considerations, had to consider such a large volume that sharp changes in density with depth have been difficult to detect. Moreover, background from multiply-scattered gamma radiation is a distorting problem that has largely been ignored by the prior art or dealt with in a comparatively inaccurate energy-discrimination scheme.